History of the Calendar

The purpose of the calendar is to reckon past or future time, to show
how many days until a certain event takes place—the harvest or a
religious festival—or how long since something important happened.
The earliest calendars must have been strongly influenced by the
geographical location of the people who made them. In colder countries,
the concept of the year was determined by the seasons, specifically by the
end of winter. But in warmer countries, where the seasons are less
pronounced, the Moon became the basic unit for time reckoning; an old
Jewish book says that “the Moon was created for the counting of the
days.”

Most of the oldest calendars were lunar calendars, based on the time
interval from one new moon to the next—a so-called lunation. But
even in a warm climate there are annual events that pay no attention to
the phases of the Moon. In some areas it was a rainy season; in Egypt it
was the annual flooding of the Nile River. The calendar had to account for
these yearly events as well.